The present invention relates to state variable, sine wave oscillators, and particularly to such oscillators employing circuits for fine tuning the frequency thereof.
A commonly known oscillator topology is the state variable oscillator in which the oscillator circuit provides an analog solution to a second order differential equation resulting in a sinusoidal output signal. The differential equation takes the following form: ##EQU1##
The instantaneous amplitude of the oscillator signal is represented by the term V; the constants a, b, and c establish the frequency of the oscillator signal. Such circuits typically employ an inverter amplifier stage followed by first and second consecutive integrating amplifier stages, the output from the second integrator being fed back to an inverting input of the inverter stage, and the output from the first integrator being fed back to the inverting and a non-inverting input of the inverter stage, each of the stages comprising operational amplifiers having associated corresponding input and feedback impedances. The state variables are a function of input and feedback resistor and capacitor values used in the amplifier and integrator stages. Oscillation occurs at the frequency at which there is unity gain in the feedback loop from the second integrator to the inverter, while the amplitude of the oscillator output is determined by the gain in the feedback loop from the first integrator to the inverter.
Non-ideal characteristics of real amplifiers and capacitors cause the amplitude of the oscillator output to be unstable and unpredictable, in the absence of external control. Leveling of the amplitude is typically accomplished by a field effect transistor in the first stage of the oscillator, the field effect transistor controlling the gain of a non-inverting input to that stage in response to an ancillary leveling circuit which monitors the oscillator output amplitude. While the gross frequency adjustment of such an oscillator is accomplished by changing the resistor and capacitor values of the integrator stages, fine tuning of the oscillator is typically accomplished by varying the input resistor value to the inverting input of the first stage from the output of the second stage, thereby changing the inverter gain experienced by that feedback loop and, hence, the frequency at which the oscillator experiences unity loop gain.
Unfortunately, fine tuning of the frequency by adjusting the input gain of the inverter stage also affects the gain experienced by the feedback from the first integrator through the non-inverting input of the inverter stage and, hence, the amplitude of the oscillator output signal. Although such variations in amplitude would ultimately be corrected by the leveling circuit, variations in the gain for purposes of tuning which occur faster than the bandwidth of the leveling circuit will result in amplitude modulation of the oscillator output, which is undesirable. Thus, there is a need for a state variable oscillator circuit that will permit fine tuning of the frequency of the oscillator without affecting the oscillator output amplitude.